LMZ10503EXT: Datasheet vs Webench

Ray Pourjalilvand

Part Number: LMZ10503EXT

Hello,

The datasheet for this device has an example for converting 5V to 2.5V (sections 8.2 and 8.3). When I enter the resistor values for Rfbt and Rfbb given in the datasheet into Webench, it complains saying that the values are lower than the minimum allowed. I have two questions regarding that:

1- Which one should I believe more, the datasheet or Webench?

2- It seems that when I change the output current from say 3A to 1A, the two resistors values for Rfbt and Rfbb change quite a bit. Why would that be the case if the output voltage remains the same? Aren't these resistors mostly used for voltage divider purposes?

Thank you in advance for your support

Ray P.

over 3 years ago

+1 Davor Pavlovski over 3 years ago

TI__Expert 3611 points

Hi Ray,

This is an issue with Webench, I will check it with our team that is supporting this.

You should always follow the datasheet for recommendations regarding the application of our devices.

Regards,
Davor

+1 Ray Pourjalilvand over 3 years ago in reply to Davor Pavlovski

Prodigy 110 points

Hi Davor,

Have you had a chance to check with your team about this issue?

Also, would you mind having someone respond to my second question as well.

Regards,

Ray P.

+1 Davor Pavlovski over 3 years ago in reply to Ray Pourjalilvand

TI__Expert 3611 points

Hi Ray,

So actually if you want to insert any value in the resistive divider group, you first have to click on the device and then go to "Choose Alternate" and then on the top of the window there is this "Edit" button which allows to change the recommended limits of the values:

So for some reason by default Webench selects only one value for this, but you can change it to whichever you want.

Yes, those resistors are used for the voltage divider. I will check with the team why they are changing with output current.

In any case, following the datasheet is the best practice.

Regards,
Davor

+1 Davor Pavlovski over 3 years ago in reply to Davor Pavlovski

TI__Expert 3611 points

Hi Ray,

So WEBENCH also changes the total capacitance and ESR of the output capacitors for a different output current.

This means that the loop compensation has to be modified - even though the output voltage stays the same. So then it makes sense for the top resistor value to change. And then also the bottom resistor will change to keep the same VOUT.
Please take a look at Table 4 of the datasheet for loop compensation component values.

Regards,
Davor

+1 Ray Pourjalilvand over 3 years ago in reply to Davor Pavlovski

Prodigy 110 points

Hi Davor,

Thanks for your support. You answered all my questions.

Ray P.

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LMZ10503EXT: Datasheet vs Webench